Joseph DeAngelo and the Golden State Killer: How a Kiwi helped solve the US' greatest cold case

"Oh, it's just a hobby," laughs Barbara Rae-Venter on her genealogy work that helped solve one of the biggest serial murder mysteries in the United States.

After 50 years living in the US, she's lost none of her Kiwi humility.

Since April's apprehension of Joseph DeAngelo, the ex-cop dubbed alleged Golden State Killer for killing 13 women across California in the 1970s and 1980s, Ms Rae-Venter's work has been the talk of law enforcement, but had not made it into the public domain.

She wasn't just showing Kiwi humility; she feared for her safety.

"They told me to beef up security on my house," she says.

But as others who had done similar work to hers came forward, Ms Rae-Venter's apprehension eased.

If there were more than one person who could use genealogy to solve crimes, she was less of a target.

And so with her blessing, Paul Holes, who also worked on the case, sent out a tweet acknowledging her crucial work in the case and she was taken aback by the response.

It was 2006 when Ms Rae-Venter first became properly involved in genealogy.

A Remuera-raised Aucklander, she moved to the US when she was 20 and had a successful career as an intellectual property lawyer.

Twelve years ago, her father had a heart attack while driving in Northland. Her mother was killed in the accident and her father wound up in Whangarei Hospital for three months.

Ms Rae-Venter came back from the US and cared for her father, shuttling between their home in Kerikeri and the hospital, paying bills and taking care of correspondence.

"In the process of doing that I discovered my grandmother's, and her father's, birth certificates in his desk," Ms Rae-Venter told Newshub from her home in northern California.

"I was astonished to find that not only was she born in New Zealand, but her father had been born in Waipu in 1862.

"I thought my grandmother was off the boat from Scotland. I had no idea."

So, on one of her trips down to Whangarei, she went a little further down to the beach town of Waipu to do some digging into her own history.

By chance, at the Waipu museum she bumped into a genealogist who happened to know her great-grandfather.

"She told me all these stories about he'd been a terrible larrikin and all the rest of it.

"We used to go surfing as kids in Waipu cove. My dad never said, 'By the way, your great-grandfather was born here.' It was really odd find all of this out."

With her curiosity piqued, she delved deeper into her family history and their emigration from Scotland to New Zealand via Nova Scotia in Canada, but dead ends along the way led her to DNA, and so became her genetic genealogy hobby.

In March last year, after helping out with other cold cases, Ms Rae-Venter received a call from Paul Holes, a retired investigator in California, asking if she would help him in one of his cases.

Without knowing it was the unsolved Golden State murders in California, Ms Rae-Venter said yes.

"Unfortunately, a few days later I was diagnosed with heart problems and I had to have surgery.

"It wasn't until November when I had recovered that I contact Paul again and asked him if he still needed help with the case.

"He said yes and we went from there.

"It's an unknown heritage case. You've got somebody, you don't know who they are and you want to identify them.

"You take a DNA sample; you extract the autosomal DNA from that. And then you [have a file made], which can be uploaded to GEDmatch and you look for matches with that file.

"From the matches you can figure out who the person is that you're looking for."

The DNA came from a semen sample left at one of the crime scenes.

Analysing old newspaper clippings, birth records and any other information available, Ms Rae-Venter began working with investigators to build a family tree.

After months of work, they began to narrow their search down to a group of men who became their primary list of suspects.

Then, after using an eye colour prediction tool on GEDmatch, Ms Rae-Venter found out the likely suspect would have blue eyes. Only one of the group had blue eyes. His name was Joseph DeAngelo, and in April he was charged with killing 13 women across California in the '70s and '80s.